Polyamide resin can express not only high strength and high rigidity but also high deflectability under loading when it is reinforced with glass fiber. Therefore, a polyamide resin composition reinforced with glass fiber is widely used as interior and exterior equipments in the fields of electric/electronic devices and automobiles. In recent years, required level of anti-vibration property is becoming high due to a demand for making thickness of the product thin particularly in the field of electric/electronic devices and due to a demand for making size of the vehicle part small. Accordingly, there has been a demand for a thermoplastic resin composition having high specific elastic modulus expressed by (elastic modulus)/(specific gravity). A polyamide resin composition generally has high water absorption rate and results in lowering of elastic modulus when it absorbs water. Accordingly, there is a disadvantage that a polyamide resin composition reinforced with glass fiber based on a polyamide 6 or polyamide 66 ingredient lowers its anti-vibration property upon water absorption. Moreover, the rate of the resin becomes low when the filling amount of glass fiber is 60% by weight or more. Accordingly, a lowering rate of strength, elastic modulus, etc. to the absolute amount of absorbed water becomes much more whereby the use as electric/electronic part cases and interior equipment and exterior equipment parts for vehicles is limited.
Further, particularly in mounting of electric/electronic parts in recent years, surface mounting system (flow system and reflow system) has been quickly spread due to miniaturization of parts as a result of miniaturization of the product size, densification of mounting, simplification of steps and reduction in costs. In the surface mounting system, since the environmental temperature during the steps becomes the melt point of solder or higher (240 to 260° C.), the resin used therefor is also inevitably demanded for heat resistance at the above environmental temperature. Moreover, in the surface mounting step, swelling and deformation of the mounted parts caused by water absorption of the resin may become a problem. Accordingly, low water absorption property is demanded for the resin used. As to the resin which satisfies such a property, 6T nylon and 9T nylon have been known and there is mentioned, for example, in Patent Documents 1 and 2 that those aromatic polyamides can be used for electric/electronic parts of a surface mounting type.
In the Patent Document 3, Nylon 66 base is copolymerized with isophthalamide component which lowers the crystallinity followed by compounding with 60% or more of a reinforcing material such as glass fiber to give a resin composition having an anti-vibration property with high resonance frequency. However, an increase in elastic modulus is not sufficient due to the component which lowers the crystallinity, and no good balance between bending elastic modulus and specific gravity for giving the resonance frequency of 200 Hz or more is achieved in a test sample shape. There is also another fear of deterioration of elongation and impact resistance as a result of addition of mica. In addition, since this resin composition results in a significant decrease in elastic modulus due to water absorption, there is a problem that resonance frequency greatly lowers in its actual use.
In Patent Document 4, there is disclosed a long-fiber polyamide molding material in which polyamide resin is combined with glass roving fibers having a non-circular cross section. However, in Examples of this patent document, glass fibers are not compounded in an amount of 60% or more. Therefore, the ratio of (elastic modulus)/(specific gravity) which is in a proportional relation with resonance frequency is not sufficiently high. As a result, although it is a molding material having high level properties in terms of strength and impact resistance, no sufficient property is expressed so far as anti-vibration property is concerned. In addition, the polyamide disclosed in the Patent Document 4 also has high water absorption rate. Accordingly, the resin molding material results in a significant decrease in elastic modulus due to water absorption. As a result, there is a problem that resonance frequency greatly lowers in its actual use.
In Patent Document 5, plural resins such as polyamide 6, polyamide 66 and non-crystalline polyamide are used not by means of copolymerization but by means of a blended base, then a reinforcing material is highly filled while the crystallinity is still kept and, further, polypropylene is added thereto in an optimum amount whereupon high resonance frequency is achieved and, at the same time, attenuation property is also imparted. However, as to the current demand for the anti-vibration, resonance frequency of 230 Hz or more is required in the test method of Patent Document 5. When a resin expressing low elastic modulus such as polypropylene is used as a component, it is absolutely impossible to satisfy the requirement. Accordingly, there has been a demand for a combination of glass fibers with thermoplastic resin which can express higher specific elastic modulus. In addition, the polyamide disclosed in the Patent Document 5 also has high water absorption rate. Accordingly, the resin composition results in a significant decrease in elastic modulus due to water absorption, there is a problem that resonance frequency greatly lowers in its actual use.